[release/7.0] Prevent warning for implicit ILCompiler reference

[sbomer]

Backport of #79240. Since I am touching this, I'm also backporting a change to the warning message from #75043.

Customer Impact

Context: dotnet/sdk#28823

PublishAot with .NET 8 SDK targeting net7.0 produces the following warning:

Set PublishAot property to true and delete explicit 'Microsoft.DotNet.ILCompiler' package reference in your project file. Explicit 'Microsoft.DotNet.ILCompiler' package reference can run into version errors.

The 7.0 ILCompiler package produces this warning. This is by design for 7.0 scenarios, but the .NET 8 SDK uses an implicit PackageReference to reference the 7.0 ILCompiler, which should not warn.

This fix removes the warning in the .NET 8 SDK scenario, and rewords the warning when using an explicit PackageReference (a 7.0 scenario) to avoid recommendation to set PublishAot, since this is only reachable when PublishAot is already set.

Testing

There are existing tests for the unchanged behavior in https://github.com/dotnet/sdk/blob/7663c1f4d5896acfb71067be3fb9510be7bde4f1/src/Tests/Microsoft.NET.Publish.Tests/GivenThatWeWantToPublishAnAotApp.cs#L266.
Tests are being added for the 8.0 behavior in dotnet/sdk#29393.

Risk

Very low. The implicit packagereference didn't exist in 7.0, so only the warning message changes when using the NET 7 SDK.

[ghost]

Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Backport of #79240. Since I am touching this, I'm also backporting a change to the warning message from #75043.

Customer Impact

Necessary to fix dotnet/sdk#28823 when targeting 7.0 from a .NET 8 SDK.

Testing

There are existing tests for the unchanged behavior in https://github.com/dotnet/sdk/blob/7663c1f4d5896acfb71067be3fb9510be7bde4f1/src/Tests/Microsoft.NET.Publish.Tests/GivenThatWeWantToPublishAnAotApp.cs#L266.
Tests are being added for the 8.0 behavior in dotnet/sdk#29393.

Risk

Very low. The implicit packagereference didn't exist in 7.0, so only the warning message changes in 7.0.

Author:	sbomer
Assignees:	-
Labels:	area-NativeAOT-coreclr
Milestone:	-

[jeffschwMSFT]

approved. please get a code review and we will take for consideration in 7.0.x

[LakshanF]

Thanks!

[carlossanlop]

Approved by Tactics (7.0.3).
Signed off by area owners.
No OOB changes needed.
CI is green.
Ready to merge.